1. Field of the Invention
The present invention relates to an optical pickup and an optical disc device which uses the optical pickup for performing a recording and a reproducing operation on an optical disc recording medium, and, more particularly, to an optical disc device suitable for a structure which corrects spherical aberration by movement of a lens of a collimator lens system in an optical axis direction of a laser beam.
2. Description of the Related Art
FIG. 1 only shows the structure of an optical system in the structure of a related optical pickup 100.
In FIG. 1, first, a laser beam emitted from a laser 102 is rotated at a polarization angle at a ½ wavelength plate/grating 103 and is divided into three beams by a grating function. Of the three divided light beams, the light beam reflected by a polarization beam splitter 104 is transmitted through an illustrated convex lens 105 and is focused on a monitor photodetector 106. A signal obtained at the monitor photodetector 106 is used to control the intensity of the light beam with which a disc 50 is irradiated.
Of the divided light beams, the light beam transmitted through the polarization beam splitter 104 is transmitted through an illustrated convex lens 110.
The convex lens 110 functions as a collimator lens. The light transmitted through the convex lens 110 is generally collimated and is reflected by a reflective mirror 113. The convex lens 110 is also called a collimator lens 110.
The light reflected by the reflective mirror 113 is transmitted through an illustrated ¼ wavelength plate 114, and, then, passes through an objective lens 115 in order to be focused as a beam spot on a recording layer of the disc 50.
Light reflected from the recording layer of the disc 50 impinges upon the polarization beam splitter 104 by being transmitted through the aforementioned path in the reverse direction. The light is reflected here, is transmitted through an illustrated multi-lens 107, and impinges upon a signal detection photodetector 108. In this case, the multi-lens 107 used has a cylinder surface.
Reflected light information obtained at the signal detection photodetector 108 is used for generating an RF signal, various servo signals, and an address signal.
An optical system such as that shown in FIG. 1 is designed so that the amount of spherical aberration becomes a minimum when the thickness of a cover layer (hereunder referred to as “cover thickness”) from a surface of the disc 50 to the recording layer is set at a value assumed to be a reference value under the condition that parallel light which has exited from the collimator lens 110 passes through the objective lens 115 in order for the recording layer of the disc 50 to be irradiated therewith.
Therefore, for example, when cover thicknesses differ according to discs 50, or when cover thicknesses at recording layers of multi-layered discs differ, spherical aberration occurs to a certain extent.
To overcome this problem, hitherto, it has been possible to correct spherical aberration when there are differences in the cover thicknesses according to discs 50 or when the discs 50 are multi-layered discs as mentioned above by forming the collimator lens 110 shown in FIG. 1 so as to be movable in an optical axis direction.
In other words, the collimator lens 110 is moved in the optical axis direction in order to move an object point of the objective lens 115 and vary a beam wave surface by this action, so that spherical aberration is corrected.
This will be described in more detail with reference to FIGS. 2A and 2B. FIGS. 2A and 2B only shows the laser 102, the collimator lens 110, and the objective lens 115 in the structure of the optical system shown in FIG. 1, and, thus, does not show the other components.
First, in FIG. 2A, the collimator lens 110 is at a reference position in a laser beam optical axis direction, and a laser beam which has exited from the collimator lens 110 is a substantially parallel light beam (light beam of an infinite system). The optical system is designed so that, in this state, the amount of spherical aberration becomes a minimum when the cover thickness of the disc 50 is a reference value.
FIG. 2B shows a state in which the collimator lens 110 is moved, for example, by a predetermined amount towards the objective lens 115 in the laser beam optical axis direction. Here, the laser beam which has exited from the collimator lens 110 is, as shown in the figure, not a parallel light beam but a converging light (light beam of a finite system).
According to the structure shown in FIG. 2B, a laser beam wave surface changes differently from a wave surface shown in FIG. 2A. This change causes the wave surface of the laser beam which has exited from the objective lens 115 to be subjected to a predetermined aberration.
Here, the amount of aberration which the laser beam is subjected to can be adjusted by the amount of movement of the collimator lens 110, so that, by the aberration, spherical aberration can be corrected in accordance with the differences in the cover thicknesses and differences in cover thicknesses according to recording layers.